1. Field of the Invention
This invention relates to a torque detection mechanism for a flexed meshing type gear drive. In particular, this invention pertains to a torque detection mechanism which is capable of compensating detection errors (rotational ripple) thereof occurring in accordance with rotation of an input shaft even when no torque is transferred. Further, this invention also pertains to a torque detection mechanism which is capable of enhancing linearity of a torque detection output.
2. Related Art Description
A typical flexed meshing type gear drive has a circular rigid internal gear, a circular flexible external gear disposed inside the rigid internal gear, and a wave generator having an elliptical shape fixed into the flexible external gear. The flexible external gear is flexed into elliptical shape by the wave generator, whereby teeth of the external gear which are located on both ends of a major axis of the elliptical shape are meshed with teeth of the rigid internal gear. These meshing portions of the teeth moves circumferentially as the wave generator connected to an input shaft rotates. As a result, relative rotation between the flexible external gear and rigid internal gear occurs in responsive of the difference in number of teeth between the both gears.
As a torque detection mechanism applicable to this type of flexed meshing type gear drive, one has been known that is constituted so that strain gauges as a torque detection element are adhered on an outer circumferential surface of a cup-shaped flexible external gear and, based on detected strain, an amount of torque transferred via the flexed meshing type gear drive is calculated.
Each part of the flexible external gear is repeatedly deformed in its radial direction by the wave generator. Thus, even when no torque is transferred, each part of the flexible external gear is deformed radially as an input shaft connected to the wave generator rotates. More specifically, each part of the elliptically deformed flexible external gear is repeatedly deflected radially at two cycles per rotation of the wave generator connected to the input shaft. As a result, an output corresponding to such deflection is inevitably obtained from the strain gauges.
In order to assure torque detection with high accuracy, it is necessary to delete output error contents (rotational ripple) due to such cyclic deflection from a detected output so as to obtain an amount of strain corresponding only to the transferred torque. The strain varies in the form of a sign wave having a phase of 180.degree., and therefore it can be deleted by a combination of outputs from a pair of torque detection elements such as strain gauges which are provided on the flexible external gear at an angular interval of 90.degree..
Conventionally, a pair of strain gauges are adhered on an outer circumferential surface of the flexible external gear so that these strain gauges are arranged at an angular interval of 90.degree. around a rotational axis of the gear drive, and a combined or synthesized output of these strain gauges is used to calculate a torque transferred through the gear drive.
It is necessary for enhancing a detection accuracy of torque transferred through the flexed meshing type gear drive to improve linearity of torque detection output as well as to make sure of deleting rotational ripple included in the torque detection output.